1. Field of the Invention
This invention relates to an improved fluid coupling device for mechanical power transmissions, either vehicular or stationary, where output angular speed or torque is systematically modulated. Specifically, the present invention relates to vehicular hydrodynamic torque converters having improved structural and hydrodynamic characteristics, and automatic transmissions that use a plurality of such hydrodynamic torque converters. However, as will be evident, the teaching presented here can be applied to power transmissions generally.
2. Background and Description of Prior Art
In transferring torque between the engine and transmission in passenger vehicles, a fluid coupling, usually a hydrodynamic torque converter, is widely used. Hydrodynamic torque converters usually perform two functions. They give a smooth coupling between the engine and transmission, with torsional damping at all engine speeds. Thus, they serve as a cushion in the driveline, protecting it from shock phenomena that might cause premature component failure, especially during transient conditions where torsional loading might otherwise exceed acceptable limits. They also give hydrodynamic torque multiplication, which is particularly helpful when accelerating from rest, passing, etc. The torque multiplication process and operation of the torque converter are simple, completely automatic and reliable, and hydrodynamic torque converters typically have anticipated service lifetimes well in excess of other transmission components to which they are connected.
A hydrodynamic torque converter typically comprises three separate independent coaxial elements that each use a plurality of flow directing blades: an impeller, or power input member, usually driven by the prime mover; a turbine or output member connected to an output shaft which usually becomes the input shaft of a subsequent geartrain; and a reaction member, commonly called the stator. The stator is generally fixed to the transmission case, sometimes via a one-way clutch, which will allow rotation of the stator blades in one direction under certain conditions.
The torque converter operates primarily as a closed or sealed system filled with hydraulic fluid, and its cover or shell, usually toroidal or doughnut-like in shape, is usually integral with or fixed to one of the three elements -- the impeller, turbine, and stator. Hydraulic fluid circulated by the impeller impacts upon the turbine, effecting rotation of the turbine and transmission of torque through the torque converter. Hydraulic fluid leaving the turbine flows back again to the impeller.
The rotation of the impeller whenever the engine is running causes it to act as a pump, delivering hydraulic fluid to the turbine. If the impeller or the turbine ingests hydraulic fluid from a central location and delivers to a higher or maximum radius in the torque converter housing or shell, it is known as a centrifugal member; if it takes hydraulic fluid to a location of lower radius, it is known as a centripetal member.
As the impeller pumps hydraulic fluid radially and axially into the turbine, it also gives the fluid a circumferential velocity component as well (swirl). This oil engages or impacts the turbine, causing it to rotate in a like direction.
The coupling between the impeller and the turbine is not strong for low impeller speeds, and this allows the vehicle to remain at rest, even though the transmission is in a drive range and the engine is running. A relatively small increase in the rotational speed of the engine, however, will make the coupling between the impeller and the turbine stronger, with more torque transmitted through the torque converter.
Torque and kinetic energy are best exchanged between the impeller and turbine when the hydraulic fluid moved by the impeller follows the contours of the turbine blades, rather than impinging upon the turbine with a high angle of incidence. The shape of the turbine blades, however, generally leaves the oil passing through it moving in a direction that is not appropriate for re-entry into the impeller, that is, in a manner that would slow the impeller and cause great energy loss.
To redirect the hydraulic fluid so that it will enter the impeller in such a manner that it will assist the engine in turning the impeller, a stator is interposed in the hydraulic fluid flow path between the turbine and the impeller. The stator redirects the fluid flow from the turbine so that the momentum of the fluid is used to increase the impeller torque. This increase in impeller torque over the torque supplied to the impeller by the engine causes the ratio between turbine torque and engine torque to increase above unity, with the turbine torque reaching two or more times engine torque when needed. This is what accomplishes torque multiplication, and operation of the torque converter at this point is known as the torque multiplication or torque conversion phase.
As the turbine speed rises relative to the impeller speed, the stator no longer functions as much as a redirecting member for the oil, and the torque multiplication phenomenon subsides. Eventually, as the turbine speed rises further, the stator (often allowed to freewheel at this point by means of a one-way clutch) plays little or no role in the torque converter, and very little torque multiplication occurs. The speed ratio i, equal to the speed of the turbine n.sub.t divided by the speed of the impeller, n.sub.p, then approaches 1:1, but for operation in a regime of acceptable efficiency, does not reach 1:1: EQU i=n.sub.t /n.sub.p &lt;1
This is known as the coupling phase of the torque converter, and operation at this point results in less energy lost in the torque converter in the form of heat than operation in the torque multiplication phase. [Ref: Mercure, R. A., Review of the Automotive Torque Converter, SAE Paper 790046; also SAE Publication AE-5, Design Practices--Passenger Car Automatic Transmissions, 1988, pp. 165-242]
Internal combustion engines only operate efficiently for certain engine speeds and throttle openings. Part of the function of any transmission is to assure operation of the engine in an efficient manner. For a given desired output power, fuel is generally best utilized when operation of the prime mover is selected to be at low engine speeds and high throttle openings, unless high output power is needed. Since much time is spent in passenger vehicles cruising at low output power, high ratio ranges are needed so that operation of the engine can proceed at low or optimum speeds for a wide variety of vehicular travel speeds.
In the past decade, a twin strategy has been used to minimize fuel consumption: [1] the use of high ratio range transmission gearing to keep engine speeds as low as possible, except when high power levels are needed; and [2] lockup of the hydrodynamic torque converter at any vehicle speeds approaching cruising speeds, generally after 40 km/hr, to prevent torque converter slip or energy loss. Lockup of a hydrodynamic torque converter occurs when the impeller and turbine are coupled directly using an independent clutch means, thus avoiding energy losses in the converter itself. This strategy has been vital in meeting fuel economy targets. The hydrodynamic torque converter may be used in the usual manner ("unlocked" or opened) at cruising speeds when torque multiplication is desired for increased torque at the driving wheels, but when torque multiplication is no longer needed, torque converter lockup recommences.
The higher transmission ratio ranges do, however, pose some problems. With the large ratio increments used in high ratio range transmissions, fuel economy comes at the expense of transmission responsiveness, since ratio changes involving unlocking the torque converter and effecting a large ratio shift will be suppressed somewhat to avoid the well known "hunting" phenomenon where repeated shifts occur back and forth between transmission ratios. The sluggish feel of the automatic transmission that results does not encourage acceptance of automatic transmission control.
This can be remedied in part by increasing the effective ratio range of the torque converter. Conventional hydrodynamic torque converters have an effective ratio range of only about 1.5-3.0 : 1. When torque converter coupling phase occurs as the vehicle gains speed, the highest speed ratio i attained is still less than one, EQU i=n.sub.t/ n.sub.p &lt;1
that is, the turbine never has a higher rotational speed than that of the impeller when the impeller is driven by the engine. A hydrodynamic torque converter that could deliver a speed ratio of greater than 1:1, e.g., 1.3:1, EQU i=n.sub.t/ n.sub.p =1.3
without significant energy losses would go a long way toward allowing operation of the vehicle with an unlocked torque converter for longer periods, giving the smooth power and torque progression that only a hydrodynamic torque converter can provide.
Sizing of hydrodynamic torque converters is another area where an improvement would open up more possibilities. Certain automatic transmissions used in locomotive and diesel truck engines use multiple hydrodynamic torque converters to replace multiple disc wet clutches or the like that are used to effect transmission ratio changes. The multiple hydrodynamic torque converters are called shifting couplings, and transmissions using this arrangement can be made to have excellent ratio range and other characteristics. They are not, however, practical for application in passenger vehicles, mostly because of the size and weight of the conventional torque converters used. If the required size of the hydrodynamic torque converters could be reduced, this type of transmission as shown in FIG. 5 below would become more practical for use in passenger vehicles.
Addressing the question of torque converter size, it is important to note that for a given impeller speed, the torque capacity Mp of a hydrodynamic torque converter is essentially proportional to the fifth power of its diameter D: EQU M.sub.p .varies.D.sup.5
This means that any decrease in diameter will drastically reduce the torque capacity M.sub.p. A twelve percent reduction in diameter D, for example, cuts the torque capacity M.sub.p in half.
One way to reduce the required size of a hydrodynamic torque converter while maintaining its relative torque capacity is to utilize the converter at higher impeller input speeds. This can be done by the use of gearing between the engine output shaft and the impeller, to double, for example, the impeller input speed relative to an impeller coupled directly to the engine. This allows use of a smaller torque converter for a given required torque capacity, because [1] the torque capacity M.sub.p of a hydrodynamic torque converter is also proportional to the square of the impeller speed n.sub.p, that is, EQU M.sub.p .alpha.(n.sub.p).sup.2
but much more significantly, [2] the torque required to transmit a given power through the torque converter is proportionally less due to the higher running speed of the impeller. Doubling the impeller input speed, for example, cuts in half the required torque M through the torque converter. This is a natural consequence of the fact that power P equals torque M times angular speed .omega.: EQU P=M.omega.
A problem in trying to increase the input speed of a conventional hydrodynamic torque converter arises because the impeller is typically part of the torque converter case. With the case rotating at increased speed, large transient bearing loads and vibration can result, given the mass of the converter shell and the oil contained therein. Fine rotational balancing of the converter case is required, and the added rotational inertia of the converter is undesirable in automatic transmissions for passenger vehicles.
Two other factors influence the torque capacity, and therefore the diameter, of a torque converter. The torque capacity is also proportional to the physical density .gamma. of the hydraulic or transmission fluid used, EQU M.sub.p .varies..gamma.
which is more or less fixed because of the many engineering requirements that must be met, like lubrication properties for high mechanical efficiency, frictional properties to enhance the operation of any friction clutches such as a torque converter lockup clutch, resistance to temperature-induced deterioration, etc. In addition, there are the inherent qualities of a particular torque converter, its internal configuration, its blading (blade orientations, shapes, profiles, entrance and exit angles, etc.), and other internal geometries. This inherent effect on the torque capacity M.sub.p is expressed by a torque capacity coefficient .lambda..sub.l, and it is usually determined empirically after building a torque converter and testing its operation. The actual torque capacity M.sub.p is a product of all the above-mentioned factors: EQU M.sub.p =.lambda..sub.1 .gamma.(n.sub.p).sup.2 D.sup.5
Another problem associated with use of the shifting coupling transmission using multiple hydrodynamic torque converters in lieu of wet plate clutch or other shifting devices is that the hydrodynamic torque converters must be selectively and individually filled and drained quickly to effect the ratio changes, and since conventional hydrodynamic torque converters have a large interior volume, this typically requires 1-1.5 seconds. This long time period may be acceptable for locomotives, but is not a fast enough response time for use in passenger vehicles. Each time a transmission ratio change is to be effected, at least one hydrodynamic torque converter must be selectively drained and another filled within a small fraction of a second. Consistent switching of the hydraulic circuits is difficult in practice.